Dr Gunasekera analysed the portrayal of sex and drug use in the top 200 movies of all time.

Films released or set prior to the emergence of HIV in 1983, animated features, those not about humans and G or PG rated, were excluded, leaving 87 for review.

The only suggestion of condom use was in Pretty Woman, when Julia Roberts' character tells Richard Gere's he will have a choice of condoms.

But in 98% of sexual episodes, which could have resulted in pregnancy, no form of birth control was used or suggested.

There were also no depictions of consequences of unprotected sex such as unwanted pregnancies, HIV or sexually transmitted infections, they said.

'Major influence'

Movies with cannabis (8%) and other non-injected illicit drugs (7%) were less common than those with alcohol intoxication (32%) and tobacco use (68%), but tended to portray their use positively and without negative consequences, the researchers said.

If it was incumbent on every screenwriter to punish his or her character for having sex with a dreadful STI, films wouldn't be very enjoyable

Edward Lawrenson, Sight and Sound magazine

Fifty-two per cent of cannabis usage was shown in a positive light and 48% in a neutral light with no negative consequences.

The study revealed that only one in four movies, including 1999's 'Sixth Sense' and 2000's Cast Away, had no negative health behaviours such as unprotected sex between new partners, cannabis use, non-injected drug use, smoking and alcohol intoxication.

Dr Hasantha Gunasekera at the University of Sydney, who led the study, said: "The movie industry influences the perception of billons of people around the world.

"The top 200 movies in cinematic history have grossed in excess of US$70 billion (£39 billion) in box office takings."

He told the BBC News website: "Obviously, we understand that the movie industry is there to entertain and to make money, and is not an instrument of public health advice.

"But we feel it is surprising that there's no attempt to reflect safe sex practices or the consequences of drug use."

He added: "With globalisation and the growth of home-based media technologies, movies are more accessible to a wider audience and there is convincing evidence that the entertainment media influences behaviour."

"The study showed there were no references to important consequences of unsafe sex such as HIV transmission, spread of STIs or unwanted pregnancy."

Dr Gunasekera added: ""The social norm being presented in movies is concerning given the HIV and illicit drug pandemics in developing and industrialised countries.

"The motion picture industry should be encouraged to depict safer sex practices and the real consequences of unprotected sex and illicit drug use."

But Edward Lawrenson, deputy editor of the British Film Institute's Sight and Sound magazine, said: "I don't think it's the role of the film industry to educate the public about public health.

"If it was incumbent on every screenwriter to punish his or her character for having sex with a dreadful STI, films wouldn't be very enjoyable."